Consider otters. They are members of the Mustelidae family (mink, stoat, weasel, marten, badger, wolverine, skunk (??), ferret, etc.). All species are semi-aquatic. The sea otter rarely comes ashore.
Although we cannot predict the future evolution of any species, since any or all otter species might re-adapt to life on land, otters appear to be in transition from land to water environment.
Consider the pinnipeds, the seals, sea lions and walruses. These too, like the otters, are carnivores and closely related to land mammals in the suborder Caniformia, which includes bears, dogs, pandas, the Mustelidae, raccoons and coatis. They too seem to be in a transition between land and water. While the bone structure of their limbs is the same as any other carnivore, externally they appear as fins.